Seoul envoy thanks Xi for 'big role' in N. Korea nuclear talks

South Korea's national security adviser on Monday thanked Chinese President Xi Jinping for his "big role" in the diplomatic process that has set up a historic summit between Donald Trump and Kim Jong Un.

Just days after he stood outside the White House and announced the planned meet between the US president and the North Korean leader, Chung Eui-yong was in Beijing to brief Xi and China's top diplomats on the fast-moving efforts to address the nuclear standoff.

Trump and Kim have agreed to meet by the end of May, although they have yet to confirm a date or time.

"The situation on the Korean peninsula has recently undergone very positive changes. President Moon Jae-in believes that the leadership of the Chinese government, especially the leadership ability of President Xi has played a big role in this," Chung told Xi at the Great Hall of the People.

Chung said Xi's "unwavering" commitment to the denuclearisation of the Korean peninsula and a peaceful settlement played a "significant" part in the recent developments.

"We are very grateful to China for its consistent position," he said.

"Once again, we expect that China will continue to play an active and leading role and the South Korean government will continue to coordinate closely with China."

Chung renewed Moon's invitation for Xi to visit Seoul. The South Korean leader visited Beijing in December on a trip aimed at thawing tense bilateral relations strained over Seoul's decision to host a US missile defence system that Beijing sees as a threat to its own security.

Xi said Chung had achieved "positive results" in his visits to North Korea and the United States last week, which led to the diplomatic breakthrough.

Xi did not say more about the North Korean nuclear crisis, but he said China's relations with South Korea have "maintained a good momentum of improvement".

Earlier, China's top diplomat Yang Jiechi told Chung that Beijing "will continue to realise the goal of denuclearisation, uphold the peaceful unification of the peninsula, and solve problems through dialogue and consultation".

China, which has repeatedly pressed the US and North Korea to hold talks, has urged Xi and Kim to hold their meeting as soon as possible.

Beijing has played a key role in implementing UN sanctions on the North, which are believed to have put immense pressure on the country's fragile economy.

China is North Korea's only diplomatic ally and its most important trade partner.

Still, some in China are afraid the country, which hosted failed six-nation talks on the nuclear issue a decade ago, could be cut out of negotiations on denuclearising the Korean peninsula.

South Korea also sent its spy chief Suh Hoon to Tokyo to brief the Japanese leadership on the North Korea situation.

Japan has welcomed the Trump-Kim meeting announcement, but stressed that Pyongyang must take "concrete actions" for talks to be meaningful.

"The recent change by North Korea is the result of the maximum pressure that Japan, the US and South Korea have imposed in cooperation," Japanese foreign minister Taro Kono said Monday after his meeting with Suh.

"(We) agreed that we will not repeat our past mistakes and continue putting maximum pressure to realise the abandonment (of Pyongyang's) nuclear and missile programme."